Enzyme inhibition

Laura Armstrong & Joe Wolfensohn

Teachers

Laura Armstrong Joe Wolfensohn

Contents

Competitive and Non-Competitive Inhibitors

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Recognising Inhibitor Type in a Rate vs. Substrate Concentration Experiment

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Recall Questions

This topic requires prior knowledge of enzymes and experiments including required practical 1. You can test your knowledge on these below.

When drawing a table of results, which variable should always go in the first column?

Why do we carry out repeats in an experiment?

What is meant by Vmax in enzyme-controlled reactions?

Topic Explainer Video

Check out this @LauraDoesBiology video that explains enzyme inhibition or read the full notes below. Once you've gone through the whole note, try out the practice questions!

If you want another explainer of enzyme inhibition, check out this video from @JoeDoesBiology and once you've gone through the whole note, try out the practice questions!

Competitive and Non-Competitive Inhibitors

What are enzyme inhibitors?

  • Enzyme inhibitors reduce the rate of enzyme-controlled reactions by interfering with enzyme activity.
  • There are two main types:

    • Competitive inhibitors

    • Non-competitive inhibitors

1. Competitive Inhibitors

  • How they work:

    • Competitive inhibitors have a similar shape to the substrate and compete for the active site.

    • They bind temporarily to the active site, preventing the substrate from binding.

    • This reduces the number of enzyme-substrate complexes formed.

  • Effect on reaction rate:

    • Increasing substrate concentration reduces the effect of a competitive inhibitor.

    • At high substrate concentrations, the reaction reaches Vmax as more substrate molecules outcompete the inhibitor.

    • More enzyme substrate complexes form and fewer enzyme inhibitor complexes will form.

Example: Malonate inhibits succinate dehydrogenase, an enzyme used during the Krebs cycle in aerobic respiration.

2. Non-Competitive Inhibitors

  • How they work:
    • Non-competitive inhibitors bind to an allosteric site (a site other than the active site) on an enzyme.
    • This alters the enzyme’s tertiary structure, changing the shape of the active site.
    • As a result, the substrate can no longer bind to the active site, preventing enzyme-substrate complexes from forming.



  • Effect on reaction rate:
    • Increasing substrate concentration does not reduce the inhibitor’s effect.
    • This is because the substrate is not in competition with the inhibitor for the enzyme's active site.
    • No matter how high the concentration of substate, some enzymes will always be inhibited by the non-competitive inhibitor.

Example: Cyanide inhibits cytochrome c oxidase, an enzyme used in oxidative phosphorylation during aerobic respiration.

Recognising Inhibitor Type in a Rate vs. Substrate Concentration Experiment

  • Competitive inhibition:
    • As substrate concentration increases, Vmax is eventually reached because the inhibitor can be outcompeted.
  • Non-competitive inhibition:
    • Increasing substrate concentration does not allow Vmax to be reached because enzyme molecules are permanently altered.

Graph interpretation:

  • Competitive inhibitors: The reaction rate gradually increases, approaching Vmax at high substrate concentrations.

  • Non-competitive inhibitors: The reaction rate plateaus at a lower level, as fewer active sites are available.

Key Terms

  • Enzyme inhibitor: A substance that slows or stops an enzyme-controlled reaction.
  • Competitive inhibitor: A molecule similar in shape to the substrate that binds to the active site.
  • Non-competitive inhibitor: A molecule that binds to an allosteric site, altering the enzyme’s tertiary structure and the shape of the active site.
  • Allosteric site: A secondary binding site on an enzyme where non-competitive inhibitors attach.
  • Vmax: The maximum rate of reaction when all active sites are occupied.

Exam Tips

When explaining the effect of inhibitors, always refer to a reduction enzyme-substrate complex formation.

Compare inhibitor types using graphs: competitive inhibition can be overcome with high substrate concentration, while non-competitive inhibition cannot.

A scientist investigated the effect of an inhibitor on the rate of an enzyme-controlled reaction.
The graph below shows the results when different substrate concentrations were used with and without the inhibitor.

Which type of inhibitor was used in this investigation? Explain how the graph shows this. (3 marks)

Practice Question 1

Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!

Practice Question 2

If you want to try out another one, check this video out and see how you do!

Required Practical 1 enzymes